It sounds like at your school kids either take AP calculus BC and AP science, or AP humanities /social science… And your son wants to take some of both. The problem you have with it is not his ability to handle the material, but how time consuming it’ll e in addition to the other AP classes he’ll be taking. In that view, AB would be a compromise for him to handle both ‘tracks’.
However, if he took BC, would he be allowed to switch into AB after the first semester if he found BC too time-consuming?
Many students compromise by cutting on sleep and that is really the worst decision possible since it harms their bodies and brains.